<?php get_header(); ?>
  <main id="content" class="content">
  <?php do_action( 'basic_main_content_inner_begin' ); ?>
    <?php if ( is_home() && 'customtitle' == get_theme_mod( 'home_h1_type', 'sitetitle' )  ) { ?>
      <div class="blog-home-header">
        <h1><?php echo esc_html( get_theme_mod( 'custom_home_h1', get_bloginfo('sitetitle') ) ); ?></h1>
      </div>
    <?php } ?>
<?php
  $s1='';
  $s2='';
  $meta_key = '';
  if (isset($_GET['order']) && $_GET['order'] == 'easiest') { 
    $order = "ASC"; 
    $s1 = 'selected="selected"';
    $meta_key = 'level';
  }
  if (isset($_GET['order']) && $_GET['order'] == 'hardest') { 
    $order = "DESC"; 
    $s2 = 'selected="selected"';
    $meta_key = 'level';
  }
?>
<form method="get" style="margin-bottom: 20px;">
  <select name="order" onchange="this.form.submit()">
    <option value="0"><?php _e( 'Все записи', 'your_theme' ) ?></option>
    <option value="easiest" <?php echo $s1?>><?php _e( 'Сначала простые', 'your_theme' ) ?></option>
    <option value="hardest" <?php echo $s2?>><?php _e( 'Сначала сложные', 'your_theme' ) ?></option>
  </select>
</form>
<?php
  $current = absint( max( 1, get_query_var( 'paged' ) ? get_query_var( 'paged' ) : get_query_var( 'page' ) ) );
  $posts_per_page = get_option( 'posts_per_page' );
  $args = array(
    'post_type'       => 'post',
    'posts_per_page'  => $posts_per_page,
    'paged'           => $current,
    'meta_key'        => $meta_key,
    'meta_value'      => ' ',
    'meta_compare'    => '!=',
    'orderby'         => 'level',
    'order'           => $order,
  );
  $query = new WP_Query($args);
?>
<?php if( $query->have_posts() ) : ?>
  <?php while( $query->have_posts() ) : $query->the_post(); ?>
    <div>
      <?php the_field('level') ?>
      <?php get_template_part( 'content' ); ?>
    </div>
  <?php endwhile; ?>
  <nav class="navigation pagination">
    <div class="nav-links">
      <?php
        echo wp_kses_post(
          paginate_links( [
            'total'   => $query->max_num_pages, // количество берем из дефолтной опции запроса
            'current' => $current, // текущая страница
          ] )
        );
      ?>
    </div>
  </nav>
<?php else: ?>
  <div class="post clearfix">
      <h2><?php _e( 'Posts not found', 'basic' ); ?></h2>
      <?php get_search_form(); ?>
  </div>
<?php endif; ?>
<?php wp_reset_query(); ?>
  <?php do_action( 'basic_main_content_inner_end' ); ?>
  </main> 
  <!-- END #content -->
<?php get_sidebar(); ?>
<?php get_footer(); ?>